~5mmol/L

Chylomicron structure1)2)3)

1) Apolipoproteins (B-48, C-II, C-III)2) Less cholesterol than LDL3) More protein than LDL

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Where are chylomicrons formed?

Intestinal mucosa

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Role of chylomicrons

Carry TAGs from diet to tissues via the lymphatics and blood

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Cholesterol form on the surface of lipoproteins

Free (unesterified) cholesterol

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Are chylomicrons more or less dense than VLDL?

Less dense

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Lipoproteins that primarily carry TAGs in the blood

Chylomicrons, VLDL

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Lipoproteins that primarily carry cholesterol in the blood

LDL, HDL

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Three lipid transport systems

1) Exogenous2) Endogenous3) Reverse cholesterol transport

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Exogenous lipid transport1)2)3)4)5)

1) Dietary fats broken down into free fatty acids, mono- and diacylglycerols by intestinal lipases (bile salts emulsify) 2) Breakdown products are taken up by mucosa, reformed into TAGs and incorporated into chylomicrons3) Chylomicrons move through bloodstream and lymphatics into tissues 4) ApoC-II on chylomicron activates lipoprotein lipase on capillary wall. Lipoprotein lipase converts TAGs to free fatty acids and glycerol5) Fatty acids enter cells, are either oxidised for energy or reesterified for storage6) Chylomicrons depleted of TAGs (chylomicron remnants) are taken in the blood to the liver, where they are taken up by RME

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Endogenous lipid transport1)2)3)4)5)

1) VLDL assembled in ER of hepatocytes on ApoB-100 scaffold2) ApoE and ApoC-II associate with VLDL in the blood.3) Lipoprotein lipase (activated by ApoC-II) converts TAGs to FFAs. This converts VLDL to IDL (intermediate-density lipoprotein).4) ~50% of IDL taken up by liver via LDL receptor that recognises ApoE on IDL. ~50% of IDL sheds ApoE, becomes LDL.5) ~70% of LDL is taken up by liver via LDL-R, ~30% enters muscle, adipocytes, adrenal glands.

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How does ApoB-100 help form VLDL?

Has binding sites for microsomal transfer protein (MTP) necessary for formation of VLDL

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Reverse cholesterol transport1)2)3)4)5)

1) HDL starts as a disc of ApoA-1 (protein) and phospholipid2) Free cholesterol is esterified, incorporated into HDL33) More esterified cholesterol binds to HDL, forming HDL24) CETP assists in the exchange of esterified cholesterol from HDL with TAGs from LDL and VLDL.5) HDL interacts with SR-B1 in the liver, delivers TAGs to the liver.

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Why might high HDL levels correlate with less heart disease?

HDL in blood picks up excess free cholesterol from arterial walls, converts them to cholesteryl esters, delivers them to the liver where they are converted to bile.